Dissecting the Unique Features of Neutrophil Glycobiology in Inflammation and Infection using Glycoanalytics

Abstract

The granulated neutrophils are front-line immune cells critical for the function of the innate immune system. Many aspects of the structure and function of the heterogeneous neutrophil glycoproteome which shows strong subcellularspecific characteristics remain unresolved. Enabled by technology advancements in glycomics and glycoproteomics, we here investigate the intriguing protein N-glycosylation of human neutrophils in the context of inflammation and bacterial infection. An under-reported class of truncated N-glycoproteins, paucimannosidic proteins, was initially discovered in sputum from pathogen-infected human lungs (Venkatakrishnan et al., Glycobiology, 25(1):88, 2015). Their structures displaying simple monosaccharide compositions i.e. Man1-3GlcNAc2Fuc0-1, the associated biosynthetic machinery involving maturation stage-specific expression of β-hexosaminidases and their preferential subcellular location in azurophilic granules of pulmonary neutrophils were demonstrated (Thaysen-Andersen et al., J Biol Chem, 290 (14):8789, 2015). Importantly, these compartment- and inflammation-associated glyco-signatures were present on intact bioactive proteins including cathepsin G, azurocidin and neutrophil elastase indicating that they are not degradation products (Loke et al., Biomolecules, 5(3):1832, 2015). Recent glycomics-based studies on isolated granules of human neutrophils confirmed that paucimannosylation is enriched on proteins residing in the azurophilic granules, but also present in other compartments. In addition, paucimannosidic proteins were shown to be preferentially secreted, but not incorporated into the plasma membrane above constitutive levels, upon inoculation of isolated neutrophils with P. aeruginosa and other virulent stimuli thereby confirming granular mobility and suggesting extracellular functions of paucimannosidic proteins. Preliminary data shows that isolated paucimannosidic proteins displayed affinities to mannose-binding lectin and showed bacteriostatic activities towards virulent P. aeruginosa supporting immune-related functions of paucimannosylation in activated human neutrophils. Interestingly, isolated neutrophils from a Sandhoff disease patient displaying a HEXB-/- genotype showed reduced protein paucimannosylation relative to an age-paired healthy individual thereby confirming the importance of β-hexosaminidases in the biosynthesis of paucimannosidic proteins. Finally, other granulated immune cells were shown to also express paucimannosidic glycans indicating that these under-reported glycoproteins are integral to the innate immune system across multiple cell types. In conclusion, we here provide insights into the intriguing features of neutrophil glycobiology by expanding our knowledge of the structure, function and biosynthesis of the spatiotemporally-regulated protein paucimannosylation in the context of inflammation and infection.